This is why I switched to Prepar3D V4.5 which uses as much of the RAM as it needs with its 64 bit programming and no matter what I fly from/to whether PMDG, Coronado, Aerosoft, FSDT no matter what. I get with my old Dell Optiplex and the AMD Radeon over 20fps and sometimes up to 40fps with default scenery and a Cessna Skyhawk. If you are able, get it. I have been able to use almost all my FSX stuff on it only exception is Feelthere and PMDG who created P3D versions of their aircraft and the FSX ones don't work in P3D. I recently uploaded scenery that I created in 2018 for FSX which sometimes max'd it out with no flickering and perfect operation into my P3D . More freeware developers and designers are updating the FSX stuff to Prepar3D V4 and V5 and uploading for free downloads. I have successfully done that with ModelConverterX (FOR MY OWN USE ONLY) Just check out Flightsim.com, Simviation or AVSIM for lots of them that have been converted with permission. Milviz, A2A, and Aeroplane Heaven migrating to MSFS have generously offered a lot of their former payware for free download at ORBX or the Milviz site too. There are posts here and at the P3D Forum.